The invention relates to a system and method for reducing noise in an implantable medical device; and, more specifically, relates to reducing noise during charging of high-voltage capacitors to prevent oversensing.
In some implanted devices, sources of noise are generated internally that may interfere with sensing of signals. Some implanted devices that deliver high-voltage shocks such as Implantable Cardioverter/Defibrillators (ICDs), for example, include inductive elements such as transformers. These inductive elements may be used to charge capacitors in preparation of high-voltage shock delivery.
To perform charging of the capacitors, current is made to flow through the transformer. When current flow is abruptly enabled or disabled, as is the case in prior art implantable medical devices (IMDs), a shift in the ground plane voltage level occurs, causing a noise spike. This noise spike may adversely affect the operation of various circuits within the IMD.
One type of circuit that is particularly impacted by noise generation includes the amplifier circuits used to sense electrical signals in a patient""s body. These amplifiers are used to sense electrocardiogram (EGM) signals in the atrial and ventricular chambers of the heart. In one instance, an EGM signal may be received by an IMD and analyzed to determine the presence of an arrhythmia such as a tachyarrhythmia or a fibrillation. This type of determination is made by detecting heart rate and/or the morphology of the cardiac signal. If an arrhythmia is detected, the IMD may select and deliver appropriate therapy, which may include anti-tachy pacing (ATP), or a high-voltage shock. Noise induced in the amplifier circuits may lead to oversensing of R and P waves, and may result in the delivery of inappropriate therapy, including painful high-voltage shocks.
One particular problematic situation involves attempting to sense cardiac signals at the same time charging of the high-voltage capacitors is being initiated in preparation for shock delivery. This may occur, for example, after the system sensed the presence of an arrhythmia, then responded by delivering ATP therapy. As is known in the art, this type of therapy may be followed by delivery of a high-voltage shock in the event the arrhythmia was not terminated by the ATP therapy. To ensure the shock may be delivered as soon as possible to prevent patient syncope, many systems begin charging high-voltage capacitors in preparing for the shock delivery at the same time the system is sensing the cardiac signals to determine whether the arrhythmia has terminated. Noise induced in the amplifier circuits by the capacitor charging operation may prevent an accurate assessment of the cardiac signal, leading to inappropriate shock delivery.
Prior art IMDs have generally prevented the foregoing situation by beginning signal sensing to detect the termination of an arrhythmia only after the cessation of capacitor charging. However, the detection process requires several cardiac cycles to complete, and can be more accurate if more cardiac cycles are available for analysis. Ideally, cardiac cycles both during and after capacitor charging would be available for analysis to detect the termination of the arrhythmia.
What is needed, therefore, is a circuit that reduces the amount of noise generated by inductive elements within an IMD. Ideally, the circuit would reduce the noise levels in the amplifier circuits so that more accurate analysis of arrhythmia termination may be performed, and fewer inappropriate shocks are delivered.
The invention is directed to a system and method for reducing the amount of noise causes by inductive elements within an implantable medical device. In particular, the invention provides a system for gradually stopping and starting the current flow within the inductive elements such as transformers that are used to charge energy storage devices such as high-voltage capacitors of an implantable cardioverter/defibrillator (ICD). This more gradual change in the rate of current flow prevents ground shifts and subsequent noise spikes within the device. This, in turn, allows cardiac signals to be sensed more accurately, preventing oversensing, and minimizing the occurrence of inappropriate shock delivery.
According to one embodiment of the invention, starting and stopping of current flow during capacitor charging is accomplished using a pulsed signal having a substantially fixed frequency and a variable duty cycle. The duty cycle is varied to gradually increase current flow to initiate capacitor charging. After charging is completed, current flow is gradually reduced by decreasing the duty cycle. In an alternative embodiment, the duty cycle may be substantially constant, with the pulse frequency being varied to increase, then decrease, current flow for capacitor charging. In still another embodiment, both duty cycle and pulse frequency may be varied.
According to another aspect of the invention, the charging of the capacitors may be interrupted at predetermined intervals to allow data communications to occur between the IMD and an external device. The circuit associated with charging of the high-voltage capacitors generates electromagnetic interference that can cause data errors during communication transmissions between the IMD and an external device such as programmer. To minimize this interference, the charging circuit may be periodically disabled during the charging process so that communications may occur. These periodic interruptions in the charging are used to initiate data communications with external devices that may be completed during one or more such interruptions.
The system according to one embodiment of the invention includes a charge storage device such as a capacitor, and a charging circuit that stores energy on the charge storage device. The charging circuit has both an enabled state and a disabled state. A control circuit coupled to the charging circuit causes the charging circuit to transition from the disabled state to the enabled state over a period of time in a gradual manner so that noise spikes are not generated. A similar mechanism is utilized to disable the charging operation.
According to another embodiment of the invention, the system includes an energy storage device, and a charging circuit coupled to store charge on the energy storage device. A control circuit is coupled to the charging circuit to cause the charging circuit to store charge at a varying charge rate, such as a gradually increasing, or a gradually decreasing rate.
Another embodiment of the invention is a method that includes the step of initiating storing of a charge on a charge storage device included within an IMD. The method further includes controlling the rate of the storing of the charge in a manner that maintains the ground plane of the IMD at a substantially constant voltage potential.
The above summary of the invention is not intended to describe every embodiment of the invention. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.